1. Naval Meteorology and Oceanography Professional Development Center Fundamentals of Weather Contents Air Masses Fronts (cold, warm, occluded, stationary) Pressure Systems Clouds Winds

2. Air Masses Air masses are large bodies of air that have similar horizontal temperature and moisture characteristics. Air masses are classified by where they originate (their “source region”). First letter: c - continental m - maritime Second letter: P - polar T - tropical cP cA cT mP mT mP mT Air masses move from their source regions. As they do, distinct boundaries form between them.

3. Fronts A “front” is defined as the boundary between two air masses. Frontal intensity is defined by the the temperature and moisture differences on either side. Air mass characteristics on either side of a frontal boundary can be very different (point “A”) or more similar (point “B”). L Cold air mass (e.g, cP) Warm air mass (e.g, mT) Cool air mass A L Cold Warm Cool B The stronger the differences on either side of the front, the more potential for severe weather.

4. Fronts L Warm Cold Occluded Stationary Cold Warm Occluded Stationary There are four types of fronts.

5. Cold fronts are associated with: Sharp temperature changes over relatively short distances Changes in air moisture content (moist before, dry after) Reduced visibility in showers Wind shifts with frontal passage Pressure changes with frontal passage Unstable cumuliform clouds and showery precipitation patterns Cold Front A cold front defines the boundary between an advancing cold air mass and a warm air mass. Cool Cold Warm Cold Front

6. Cold Front Cold fronts are further characterized by their speed of movement - which defines their slopes. Slow moving cold fronts: Most clouds and weather are at and behind the advancing cold front Longer periods of rain/snow, less thunderstorm activity Fast moving cold fronts (steeper slope): Most clouds and weather are near and ahead of the advancing cold front Rain/show showers (sometimes heavy), more thunderstorm activity Thunderstorms often form ahead of front Fast-moving cold front Slow-moving cold front cold

7. Warm Front Cool ColdWarm Front Warm fronts are associated with: Extensive cloud activity ahead of the front. Temperature rises with frontal passage. Wind shifts with frontal passage. Poor visibility at and ahead of the frontal boundary. Thick, stratiform clouds and steady precipitation patterns. Overall improvement in weather conditions with frontal passage. A warm front defines the boundary between a retreating cool air mass and an overriding warm air mass.

8. Warm Front Clouds and weather are at and ahead the advancing warm front. Precipitation consists of steady rain or snow and usually no thunderstorm activity - although thunderstorms may be embedded within the frontal area and hard to discern on satellite pictures. Fog is frequently found in the cooler air ahead of the warm front. Warm fronts have extremely shallow slopes. Warm front Fog warm cool

9. Occluded Front Occluded fronts are associated with: Both warm front and cold front weather characteristics The worst weather with an occluded front is located where the cold and warm fronts meet at the surface: the triple point. An occluded front defines the portion of frontal area where the cold front has overtaken the warm front and pushed it aloft. “Triple point”

10. Occluded Front There are two types of occluded fronts: warm, and cold. Warm occlusions: Milder maritime polar (mP) air overtakes colder continental polar (cP) air. Warm occlusion weather is similar to that of a warm front. More steady, less showery precipitation. Cold occlusions: Colder cP air overtaking milder mP air. Cold occlusion weather resembles warm frontal weather before the front passage, and cold frontal weather during and after passage. Cold occlusion Warm occlusion cool cold cool warm

11. Stationary Front A stationary front has essentially no movement (the advancing cold front has “stalled out”). Cool Warm North Stationary fronts are associated with: East-west orientation. Normally clear to partly cloudy skies. Normally little or no precipitation.

12. Stationary Front Stationary fronts: Normally have “good” weather associated with them. Exceptions: If a new pulse of cold air moves in from the north, the cold front can begin to advance and a new low can form on the frontal boundary. If warm, moist air overruns the frontal boundary, widespread cloudiness and light precipitation can cover a vast area. Stationary Front Warm moist Overrunning

13. Pressure Systems There are two types of pressure systems: Highs and Lows A “high,”or anticyclone, is an area of high pressure around which the winds blow clockwise in the northern hemisphere (counterclockwise in the southern hemisphere.) High pressure is associated with sinking, more dense air. A “low,”or cyclone, is an area of low pressure around which the winds blow counterclockwise in the northern hemisphere (clockwise in the southern hemisphere.) Low pressure is associated with rising, less dense air. H L

14. Pressure Measurements The amount of force exerted by air molecules over a given area of the earth’s surface is called atmospheric pressure (or “air pressure.”) When the density of the air increases, pressure goes up. When density decreases, pressure goes down. Barometers are used to measure pressure in different units: Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa) (1 Hectopascal = 1 millibar) Inches of mercury - measured to the 1/100 (ex. 29.92 ins) The most common type of barometer is called an “aneroid barometer.”

15. Pressure Systems High pressure is normally associated with “good” weather: Clear or clearing skies, no precipitation, light winds (away from terrain effects). Low pressure is normally associated with “unsettled” weather: Cloudy skies, precipitation, gusty winds. Barometers are used to measure pressure in different units: Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa) (1 Hectopascal = 1 millibar) Inches of mercury - measured to the 1/100 (ex. 29.92 ins)

16. Pressure Systems: Isobars Isobars are usually drawn in 4 hPa increments. Denoted by a solid black line, labeled as shown. The highest and lowest pressure values within highs and lows are are depicted next to the “H” or “L” label. H L 96 00 04 08 12 20 24 16 994 1025 Lines of equal pressure are called “isobars.” 1008 hPa 996 hPa

17. Pressure Systems and Fronts Pressure systems and fronts have a direct relationship (time) L (Cold) (Warm) (Cool) As the system develops, the position of the low moves away from the cold and warm fronts. H L Cold air mass (e.g, cP) Warm air mass (e.g, mT) Cool air mass A low pressure area forms where the cold and warm front meet. H High pressure defined by the air mass “moving in”

18. Pressure Systems New Lows frequently form at the “triple point.” L L The “original low” fills and a new system moves off to begin the cycle again. L L New Low This low “fills” (dissipates) over time (time) H L Original Low (time) H

19. Wind Wind is air in motion relative to the earth’s surface. In meteorology, wind is the observed effect of horizontal transport of air masses over the Earth’s surface. It is caused by temperature differential between 2 areas. Wind speeds are plotted on meteorological charts as follows: Flag = 50 kts Long line = 10 kts (8-12 kts) Half line = 5 kts (3-7 kts) 5 kts10205065100

20. Wind Speed Scales - Beaufort Scale < The Beaufort Scale defines seas (and sea state) according to wind speeds

21. Wind Speed Scales Wind speeds not associated with tropical systems (World Meteorological Organization). 4 - 27 kts = breeze (light, gentle, moderate, fresh, strong). 28 - 33 kts = near gale. 34 - 47 kts = gale (gale, strong). 48 - 63 kts = storm (storm, violent). 64 kts and greater = hurricane force. Wind speeds associated with tropical systems. Less than 34 kts = Tropical Depression. 34 - 63 kts = Tropical Storm. 64 - 129 kts = Hurricane. Greater than 130 kts = Super Hurricane.

22. Pressure Systems: Isobars H L 96 00 04 08 12 20 24 16 994 1025 Wind barbs define wind direction and speed on a synoptic chart... …and also help define frontal boundaries

23. Pressure Systems, Fronts, Isobars, Winds: Example

24. Clouds Clouds are: Water molecules suspended in the atmosphere. Three things are required for cloud formation: - Moisture - Cooling - Condensation nuclei (something for the moisture to condense on) There are three general types of clouds: Cumuliform Stratiform Cirriform

25. Cumuliform Clouds Cumuliform clouds are unstable, vertically developed, and have generally distinct edges. They are formed either by convective action (daytime heating) or mechanical lifting (cold front). Showery precipitation is associated with cumuliform clouds. Cumulonimbus clouds are clouds with extreme vertical extent and are associated with heavy precipitation and thunderstorms. Stratiform clouds are stable and form indistinct layers. Steady, light precipitation is associated with stratiform clouds. A particular form of stratiform clouds, nimbostratus, is associated with heavy, steady precipitation. Fog is nothing more than a form of straitiform clouds (stratus) that has reached the ground. Cirriform clouds are located at higher altitudes and are composed completely of ice crystals.

26. For meteorological purposes, the atmosphere is divided into three levels (“etages”). Low etage - Surface to 6500 ft (middle latitudes) Middle etage - 6500 ft to 23,000 ft High etage - 16,00 ft to 43,000 ft Different cloud types are associated with the low, middle, and high etages. More common cloud types are shown below. Some “stay” in their etages, some extend through one or more (*): LowMiddleHigh Cumulus, Cumulonimbus *AltostratusCirrus StratocumulusAltocumulusCirrocumulus Stratus Nimbostratus * Cirrostratus (Cumulonimbus begins in the low etage and builds into the mid and high etages) (Nimbostratus frequently begins as a “mid” cloud and descends into the low etage) Clouds and the Atmosphere

27. Clouds and Fronts - Example L Cirrostratus Altostratus Nimbostratus Stratus (fog) Altocumulus Altostratus Cirriform Stratocumulus Cumulus Stratocumulus Cumulus Cumulonimbus Altocumulus Cirrocumulus

28. The End…